1. Field of the Invention
Embodiments of the present invention generally relate to wireless data transmission systems. Particularly, the present invention relates to methods for increasing the reliability of detected electromagnetic waves. More particularly, the present invention relates to methods for reducing noise in a detected electromagnetic wave used to telemeter data from a drilling operation.
2. Description of the Related Art
Many operations in the drilling, evaluation, completion, and producing of oil and gas wells require communication between the surface of the earth and various devices located within the well borehole.
During the drilling operation, various formation and drilling parameters including, among others, the weight on the drill bit, downhole pressure, and borehole directional orientation, are commonly monitored. It is known to include instruments in the drill string to measure these various earth formation properties as the drill bit penetrates the formation. This is commonly referred to as logging-while-drilling (“LWD”) or measurement-while-drilling (“MWD”). In addition to making measurements, these processes also involve the transmission of the measured parameters to the earth's surface.
A variety of data transmission systems have been used to transmit data between downhole and surface equipment. Probably the most common data transmission system is a “hard wire” or wireline system using conductor electrical cable, coaxial cable, or fiber optic cable. Generally, a wireline system includes a measuring instrument or “tool” conveyed along a borehole using a cable made of a single electrical conductor, multiple electrical conductors, one or more fiber optic conductors, or combinations thereof. The cable also functions as a means for communicating between the downhole tool and equipment at the surface. Data transmission rates can be very high using the hard wire transmission system. However, wireline systems, in general, can not be used during drilling operations. Therefore, wireline logs are typically run after a well has been drilled, or at intermediate steps in the drilling of the well with the drill string removed from the borehole. Other disadvantages of the wireline system include damage to the cable due to exposure in the wellbore, reduction of operating clearances for other operations, and complication of the tubular connection process.
Wireless systems for transmitting data have been developed to overcome the problems encountered with the wireline system. One example of the wireless system is the “mud pulse” system, which utilizes the drilling fluid as the transmission medium. As the drilling fluid is circulate in the wellbore, the flow of the drilling fluid is repeatedly interrupted to generate a varying pressure wave in the drilling fluid as a function of the downhole measured data. A drawback of the mud pulse systems is that the data transmission rates are very slow. Transmission rates are limited by poor pulse resolution as pressure pulsed attenuates along the borehole and by the velocity of sound within the drilling mud.
Another wireless data transmission system used in the industry is an electromagnetic (“EM”) telemetry system. FIG. 1 shows a conventional EM telemetry system used in a drilling operation. The drill string 10 is separated into two portions by an insulating gap ring 32. The lower portion of the drill string 10 may include a drill bit 20, EM transmitter 35, and other instruments such as a MWD tool 30. Drilling and geological data acquired by the MWD tool 30 is relayed to the transmitter 35, which broadcasts the data in the form of a low frequency EM wave 15. Specifically, voltage applied between the upper and lower portions of the drill string 10 generates the EM wave 15 that propagates outward and upward towards the surface 3. The EM wave 15 travels through the earth to the surface 3 where a receiver 70 may measure the potential difference between a receiver 60 located proximate the blow out preventer 25 and an antenna 50 located at a predetermined distance away from the blow out preventer 25.
Date transmission via an EM telemetry system relies on the accurate detection of weak electrical signals by the receivers located at the surface. As such, stray surface noises or other ambient noises may interfere with the accuracy of the detected signals. For example, the rig 5 itself may produce noises having a frequency of 60 Hz. Thus, data extracted from the EM signal using the EM system in FIG. 1 may be compromised by the rig noise. However, because rig noise frequency is typically higher than the EM wave, this noise may be easily removed by using notch filters.
Another source of noise arises during drilling. Drill pipes are often inspected before deployment. Part of the inspection process may require the drill pipe to be magnetized. It is believed that electrical induction from rotation of magnetized drill pipe adjacent the BOP may produce a low frequency noise. Unlike rig noise, drill pipe noise may fall within the same frequency band as the EM signal, thereby making noise rejection more difficult.
There is a need, therefore, for a method for reducing the signal noise in a detected EM wave used to telemeter data during wellbore operations. There is also a need for a method for increasing the accuracy of telemetering data in real time between the surface and a downhole equipment.